Study Of Light-Driven CO₂ Assimilation In Photosynthesis

Photosynthesis is a process in which green plants（including algae）use light energy to convert carbon dioxide（CO₂）and water（H₂O）into energy–rich organics and meanwhile release oxygen（O₂）.Photosynthesis is the most important chemical reaction on the Earth and is the foundation of various life activities.According to the principle of photosynthesis,CO₂assimilation refers to the reduction of CO₂ into organics during the stage of dark reaction,which is also known as"CO₂ reduction"or"CO₂ fixation".Although the conclusion has been accepted by most researchers,there are still a lot of puzzles while some scientists hold that CO₂ assimilation may also occur in the light reaction stage.Unfortunately,sufficient experimental evidence has not been collected so far.Then,can CO₂ assimilation take place during the light reaction stage,and what is the location if it can happen,and what are the assimilation pathway and products?Without any doubt,above scientific issues are of great significance for study.However,relevant research is extremely scarce at current stage.Aiming at above issues,this thesis selected specific biological systems（algae and higher plants）as the research object,and demonstrates that there is a light–driven CO₂ assimilation（hereinafter referred to as"CO₂ photoreduction"）during the light reaction stage.Then,this CO₂ photoreduction in photosynthesis was studied through designing and carrying out a series of control experiments combined with in–situ mass spectrometry and isotope labeling,finally with the location and products of CO₂ photoreduction preliminarily confirmed after improving relevant analysis methods.The main research contents and conclusions of this dissertation are summarized as follows:（1）By jointly using in–situ mass spectrometry（MS）,gas chromatography（GC）and isotope labeling,the photoreaction of photosystem II（PSII）core complex was studied.The results show that during the light reaction,PSII core complex not only releases O₂,but also generates C1 compound of methanol（CH₃OH）.By precluding the possibility that CH₃OH stems from photorespiration or the decomposition of cell wall pectin demethylation,it is verified by¹³CO₂ and C¹⁸O₂ labeling experiments that CH₃OH comes from CO₂ photoreduction.This study reveals that CO₂ reduction can take place during the light reaction stage,namely there is a light–driven CO₂ assimilation that occurs on PSII core complex.Further,it is speculated that this CO₂ photoreduction behavior may be synchronized with CO₂ assimilation under the dark reaction,with related mechanism to be further studied.（2）The analytical method of glycolaldehyde（GA）,an important C2 compound,was improved in order to analyze and master the product information of CO₂ photoreduction more comprehensively,and it was confirmed that GA widely exists in different types of plants.Firstly,considering the needs of liquid chromatography,the existing forms of glycolaldehyde in methanol and the dynamic equilibrium between different components were studied experimentally and theoretically.The results demonstrate that the equilibrium component of GA in methanol solution is different from that in aqueous solution,and GA exists in the equilibrium form of five dimers and two monomers in methanol solution,among which the glycolaldehyde hemiacetal（GAHA）accounts for a larger proportion（>90%）.Secondly,the plants with different CO₂ assimilation pathways were systematically analyzed.It was found that GA widely existed in various green plants such as Chlorella,spinach,maize and Aeonium decorum f.variegate of the genus Aeonium.This part of work provides an important analytical method support for the study of CO₂ photoreduction products.（3）To further study the products of CO₂ photoreduction,the source of aldehyde sugars such as GA was explored,and it was confirmed that they were CO₂ assimilation products of photosynthesis,namely from CO₂.In this part,Chlorella was selected as the research object.Based on the key chemical reaction of life origin（Butlerow reaction）and the fact that glycolaldehyde generally exists in plants,the existence of relevant aldose compounds and their possible relationship with CO₂ photoreduction were analyzed and determined by combination with in–situ MS,chemical derivatization,isotope labeling and high performance liquid chromatography–mass spectrometry（HPLC–MS）,etc.The results suggest that several aldehyde sugars such as GA exist in the products of Chlorella photosynthesis.Furthermore,¹³CO₂ isotope labeling experiment confirm that the aldehyde sugars come from CO₂.This study suggests that in plant photosynthesis CO₂ may be reduced to aldehyde sugars such as GA during the light reaction stage.We speculate that this new CO₂ assimilation pathway is similar to Butlerow reaction in mechanism,which needs a further investigation.